Electronics have become more miniaturized and sophisticated in recent years. However, along with sophistication and miniaturization comes the necessity for better noise shielding.
Noise immunity is extremely important in printed circuit boards for use in communication equipment. Crucial parts of the board must be shielded against noise and RF interference. Shielding also provides some immunity from unauthorized reception by sophisticated surveillance equipment. The perfect shield might be a grounded, sealed box, covering the whole unit. However, this would not allow for power signal access and therefore would not be a usable assembly.
Another necessary attribute of modern circuit boards are that they be of the multi-layer type, in order to facilitate multiple connections between various components. Multi-layer circuit boards have a plurality of layers, each of which is formed with a number of traces. The components on the board face are connected with one of the layers, and therefore connected by the traces on that layer and the traces on no other layers. Each of these layers also has a ground plane.
One standard way of shielding against noise has been to provide a ground plane. Such a ground plane enables ready connection to "ground" which in theory has no noise. However, the way in which the ground planes were treated in the prior art was relatively deficient.
One attempted RF shielded enclosure is shown in U.S. Pat. No. 4,658,334 to McSparran. This technique provides an RF shield around the circuit board at top and bottom and also shows putting an RF shield 104 on the edges of the multi-layer board. The RF shield on the edges, however, does not necessarily make good contact with these edges. Moreover, the shielding enclosure of McSparran is complicated as it must be located on top and bottom and must be screwed and unscrewed in order to reach the board.
Another attempted solution is shown in U.S. Pat. No. 3,179,913 to Mittler. FIG. 21 of Mittler shows a multi-layer matrix board which uses edges formed of a ground plane in order to attempt to shield the device. A plurality of these devices are located in racks, and multiple layers of the board are held together by screws. Therefore this system is hard to use. It is an object of the present invention to provide an easy-to-assemble and use system which enables ready access to the components on the circuit board but yet provides a maximum shielding for the circuit board.